How can I tell him. Please help!?

Two lessons to be learn here. 1) The grass is not always greener on the other side. The older boy may be new and intriguing but not always the best choice. 2) Be prepared to end up alone. When you make these choices you need to make sure you fully understand that the consequence can be that you end up losing both. If you are determined to be with Colin then you need to just let Andy know that you are just not feeling the same as you once were and that you are sorry. If you are sympathetic to his feelings and you don't flaunt Colin around, even though he may be mad at first he will come around. Especially if you still call and talk with him about others things going on in your lives. Good Luck and just be understanding.